1. Field of the Invention
The present invention relates to a lighting device for a display device and a display device using the same.
2. Description of the Related Art
In a display device having non-luminous optical elements as typified by a liquid crystal display device, a backlight device is provided on the backside of a display panel such as a liquid crystal panel, so as to illuminate the display panel.
Conventionally, a voltage-controlled dimming system or a PWM (Pulse Wide Modulation) dimming system is used as a dimming system for a backlight. The voltage-controlled dimming system controls the voltage applied to a fluorescent tube via an inverter for dimming purposes, by which a wide dimming range cannot be provided.
On the other hand, the PWM dimming system periodically turns a light source on and off for dimming purposes, while controlling the time ratio between light-on periods and light-off periods. This system can provide a wide dimming range by adjusting the on-off time cycle. Therefore, many liquid crystal display devices employ the PWM dimming system for a backlight.
In recent years, techniques for using a plurality of PWM signals in combination have been also developed. For example, in order to improve visibility of moving images, JP-A-2006-53520 discloses a technique that generates a new periodic signal by combining two kinds of PWM signals, i.e., a PWM signal for setting light-on periods and a PWM signal for setting light-off or black insertion periods, by which an on-off operation of a light source is repeated during each light-on period.
In a backlight device thus using the PWM dimming system for dimming control, beat tones may be generated during the dimming control of fluorescent tubes, due to the second and third harmonics of a dimming control frequency. The beat tones may cause discomfort for a user, and therefore generation thereof must be suppressed.
The beat tones may result from various factors. The factors include an abrupt increase of a tube current flowing through a fluorescent tube and a momentary flow of an excitation current immediately preceding turn-on, which cause magnetostriction (called a whisker voltage), known to contribute to generating beat tones (as shown in JP-A-2004-55431, for example), when the fluorescent tube turns from OFF to ON.
According to the invention described in JP-A-2004-55431, elimination of beat tones can be achieved by a dimming means that controls the pulse rise time of PWM signals in order to suppress the whisker voltage.
However, the control of the pulse rise time of PWM signals fails to achieve adequate elimination of beat tones. Therefore, the factors contributing to generating beat tones may include factors other than the whisker voltage, and the inventor of the present application discovered that periodical vibration of a chassis covering light sources contributes to generating beat tones.